Thread: V8 extractors - worth it or not?

That makes some sense to me - high revs = high HP but torque is at lower revs so some restriction would make sense just working out the right amount seems to be giving me a head ache.

This pans out with huge valves and torque cams dont really go together

Originally Posted by incisor

we were always taught that 4 stroke engines need some backpressure to scavenge properly if you wanted torque...

i cant for the life of me remember the reason....

Scavenge happens when there is lower pressure in the exhaust and it sucks out the burnt fuel air mix, does it not? Back pressure won't help there, I would have thought.

Originally Posted by blitz

That makes some sense to me - high revs = high HP but torque is at lower revs so some restriction would make sense just working out the right amount seems to be giving me a head ache.

This pans out with huge valves and torque cams dont really go together

Oversized valves can give less power / torque , particularly at low revs. The problem being that the larger valves give lower air velocity, which is an important consideration.

I had this problem years ago in a Holden 253 which had big valve heads. Pulled really well over 3000 rpm, dead below.

Originally Posted by blitz

things got a bit out of agreement at that point but general consensis was because it burns out valves just the level was argued about. And no-one could clearly explain why huge exhaust would make rice burner go slower but all agreed it would??

I would hazard a guess that the lower back pressure would allow unburnt fuel / air mix to exit the exhaust valve during the overlap period (when both inlet and exhaust valves are open). that would leave less fuel / air in the cylinders to burn, giving less power.

That is why a major exhaust change might require different camming, etc.

Originally Posted by BigJon

I would hazard a guess that the lower back pressure would allow unburnt fuel / air mix to exit the exhaust valve during the overlap period (when both inlet and exhaust valves are open). that would leave less fuel / air in the cylinders to burn, giving less power.

That is why a major exhaust change might require different camming, etc.

Ok how does this sound - for an engine to work at it's optimum there would have to be some sort of volumetric efficientcy ratio between the size of the exhaust and the inlet. so if the exhaust was to small it would be to restrictive and if it was to big it would loose it's scavenging effect? so both sides would have a detrimental effect on efficency.

Therefore one would have to work out the flow rate of the exhaust based upon the CFM of the intake to determine the optimum size, probably at the highest torque point in the rev range, using basic physics here.

Does that make sense or did I just make it more confusing

Also bearing in mind that the intake is drawn in with only atmospheric pressure behind it (assuming not turbo charged or supercharged), but the exhaust is pushed out by the piston moving upwards.

IIRC, volumetric efficiency is just the measurement of how much air / fuel mixture gets into a cylinder, compared to how much will fit. If it is a 1 litre cylinder (to make the numbers easy) and you can draw in 500cc of air (at atmospheric pressure) during the intake stroke, then the VE is 50%. If you draw in 750cc, then VE = 75%. If you turbocharge and cram in 1250 cc of air (ie, above atmospheric pressure) then your VE = 125%. One of the few times when you can have something greater than 100%.

I hope I have this correct, it has been too many years since I was at Trade School learning this stuff.

Originally Posted by BigJon

Scavenge happens when there is lower pressure in the exhaust and it sucks out the burnt fuel air mix, does it not? Back pressure won't help there, I would have thought.

in a 4 stroke the spent charge is pushed out by the piston moving back up before the valves alter and the next downward movement draws in another charge, lower pressure in the exhaust has very little to do with it if you are after torque as the positive displacement moves all you need moved, attaining hi revs is a different story all together or so i was taught when i was getting my ticket..but i am old school, theory could well and truly have changed since then.